Stabilized zirconia powders are widely used to provide thermally stable and abrasion resistant coatings to parts that are exposed to very high temperatures during use but which are also exposed to ambient temperatures. It does, however, have a well-known drawback in that, as it cycles between high and low temperatures, it undergoes a crystal phase change from the tetragonal crystal phase structure, which is stable at elevated temperatures, to the monoclinic crystal phase structure, which is stable at room temperature. Volume changes occur as this crystal phase change takes place compromising the physical integrity of the zirconia coating. There is another phase of zirconia which is also stable at temperatures above the monoclinic/tetragonal transition temperature, (the “cubic” phase), but since little or no volume change occurs on the transition from cubic to tetragonal, this is treated for the purposes of this Description as a form of the tetragonal phase and is not distinguished therefrom.
In order to resolve the integrity problems with zirconia coatings resulting from the crystal phase changes, it is common to use stabilized zirconia in powder coatings. Stabilization can be achieved by the addition of a number of additives that have the effect of inhibiting the conversion from the tetragonal crystal phase to the monoclinic crystal phase upon cooling. Such additives include stabilizing oxides such as calcia, magnesia, yttria, ceria, hafnia, and rare earth metal oxides.
Stabilized zirconia coatings are widely used to produce an abradable protective coating on surfaces or thermal barrier coatings. They are typically applied as sprays by a flame spray or a plasma spray approach.
In the production of stabilized zirconia powders, the most common technique is described in U.S. Pat. No. 4,450,184 to Longo et al., in which an aqueous slurry comprising a blend of zirconia and stabilizer materials is fed into a spray dryer to form dried porous particles. The porous particles are fused into homogenous hollow structures with a plasma or flame spray gun which melts and fuses the components such that the particles ejected therefrom are stabilized zirconia. Thermal spraying of the hollow spheres creates a porous and abradable coating. However the Longo process does not achieve a high degree of uniformity of composition.
U.S. Pat. No. 5,418,015 to Jackson et al. discloses a feed composition for thermal spray applications composed of stabilized zirconia mixed with zircon and a selected oxide to form an amorphous refractory oxide coating. Such products do not however have the required level of size and compositional uniformity that would be desirable to secure good thermal barrier coating compositions for high temperature applications. This is at least in part because there are many opportunities for variability in the resultant coating as a result of differing particle sizes in the feed, the flame or plasma gun design/shape, feed rate pressures, and the like.
Another method of forming stabilized zirconia involves sintering wherein the components are blended together as powders, sintered, and upon cooling, the sintered mass is broken up into particles. These particles may then serve as feed for a flame spray device. Unfortunately, this process does not provide for a high level of chemical homogeneity in the stabilization and results in widely varying shapes and particle sizes in the feed.
Ceramic mixtures such as stabilized zirconia may also be made by electrofusion. The fused mixtures are much more uniform than those made by the processes discussed above because they are the result of complete melting of the components. However, the components are difficult to melt and have poor flow characteristics as a result of their high density and irregular shape generated when the fused masses are crushed to provide particles. Thus, the currently available stabilized zirconia powders made by electrofusion have a high degree of un-melted material in the spray process resulting in poor efficiencies and coatings with a high content of such un-melted material particles. The un-melted particles introduce stresses into the coating due to the varying density of the coating in and around the un-melted particles. As a result, the longevity of the resultant coating is diminished, particularly under stressful conditions.
Notwithstanding the state of the technology, it would be desirable to provide a ceramic powder having a high level of chemical and morphological uniformity, which in turn, provides a durable thermal spray coating.